The photons emitted by galaxies (from stars and accreting super-massive blackholes) generate an uniform background know as the extragalactic background light (EBL). The spectrum of EBL is an important quantity for studying several astrophysical problems. For example, in extreme UV regime, the EBL is essential to study the hydrogen and helium reionization, metal enrichment and thermal history of the intergalactic medium, and missing baryons at low-redshift. In optical to far-IR regime, the EBL gives insight into the propagation of the high-energy gamma rays emitted by distant sources. In this talk, speaker will discuss how to count photons and estimate the spectrum of the EBL, especially the theoretical and observational intricacies involved in it. Also, he will introduce the state-of-the-art EBL model and its applications to wide range of problems.